I always assumed he had an inner hollow because he started turning into a hollow while trying to recover his death god powers.
That really is the most logical option to think, even though it is a theory, it really has logic to it.
I mean, when Ichigo was gaining his shinigami powers back, the chain of fate nearly eroded, and the
hollow mask came first.
Then he fought it off and the mask kept coming back.
Even though this is a theory, seems
very logical to me.
P.S This doesn't really need a tag, since all this shit is in like, episode 19 - 30.